Facts: This statement implies that something that occurs naturally can’t be harmful, or must be less harmful than a human-made hazard (a common misperception). It also suggests that since methylmercury occurs naturally, it has always been in fish (true), and therefore is not worth worrying about (not true). Yes, there has always been naturally-occurring mercury in fish, but that does not make it less toxic. Many natural, poisonous substances occur in foods and have been subject to risk-management measures once their hazards became evident—lead, arsenic, and aflatoxin for example. Methylmercury made in nature is just as toxic as methylmercury made in a laboratory, and efforts to minimize exposure to it are fully justified.
The fact that much of the mercury in the oceans occurs naturally has had one important practical impact: Natural contaminants are exempt from the labeling requirements under California’s Proposition 65, which has allowed the tuna industry to avoid having to put mercury warnings on can labels, up to this point. But in terms of health hazards, whether the mercury in fish is natural or not makes no difference at all.